Denis Avey, even at the age of 91, cuts a formidable figure. More than 6ft tall, with a severe short back and sides and a piercing glare, he combines the pan-ache of Errol Flynn with the dignity of age. This is the former Desert Rat, who, in 1944, broke into — yes, into — Auschwitz, and he looks exactly as I expected. He removes his monocle for the camera, and one of his pupils slips sideways before realigning. It is a glass eye. I ask him about it. He tells me that in 1944, he cursed an SS officer who was beating a Jew in the camp. He received a blow with a pistol butt and his eye was knocked in.

If Avey’s story is difficult to believe, it is worth bearing in mind that it is not without precedent. In 1944, the British PoW Charlie Coward, a sergeant-major from the Royal Artillery who had attempted escape 14 times, infiltrated the camp dressed as a Jewish prisoner to gather intelligence from a British Jewish naval doctor interned there. After the war, Coward testified at the IG Farben trial in Nuremberg. His life story was made into a film The Password is Courage in 1962, starring Dirk Bogarde.

Yad Vashem, the Israeli Holocaust authority, is in the final stages of researching aspects of Avey’s story with the intention of granting him the title of Righteous Among the Nations. “For obvious reasons this honour cannot be based on Avey’s word alone,” says Susan Weisberg, spokeswoman for Yad Vashem. “Each case must be substantiated by eyewitness testimonies and archival documents of the period.”

Avey, born in 1919 on an Essex farm, lived a rough-and-tumble lifestyle and grew into a daredevil. “I once jumped from a branch 45ft high, just for the thrill of it,” he says. “I had a shock of red hair and a temperament to match.”

He also had an affinity for the underdog. As head boy of his school, he used his physical strength to protect the weaker boys. “If there is one thing I’ve always abhorred it is bullying,” he says. “I could dish it out back then. Legislation wouldn’t let me now.”

These traits would serve him well at war. In 1939 he volunteered for the Army — because he was too impatient to wait a week for the RAF. “I ended up in the 7th Armoured Division, the original Desert Rats,” he says. “We operated behind enemy lines in Egypt. In 1942 we were ambushed. I was wounded and taken prisoner by the Germans.”

Avey was a troublesome prisoner. In the summer of 1943 he was deported to Auschwitz, in Poland, and interned in a small PoW camp on the periphery of the IG Farben factory. The main Jewish camps were several miles to the west. “I’d lost my liberty, but none of my spirit,” he says. “I was still determined to give as good as I got.”

But he knew immediately that this was a different order of prison. “The Stripeys — that’s what we called the Jewish prisoners — were in a terrible state. Within months they were reduced to waifs and then they disappeared. The stench from the crematoria was appalling, civilians from as far away as Katowice were complaining. Everybody knew what was going on. Everybody knew.”

Remarkably, Avey was able to think beyond the war. “I knew in my gut that these swine would eventually be held to account,” he says. “Evidence would be vital. Of course, sneaking into the Jewish camp was a ludicrous idea. It was like breaking into Hell. But that’s the sort of chap I was. Reckless.”

According to the historian Sir Martin Gilbert, Avey’s hunch was right. “Auschwitz would not become known as a place of extermination until the spring of 1944,” he says. “When the world found out, there was outrage. After the war, British war crimes investigators were desperate to find PoWs with information about the camps.”

Avey’s audacious plan was made possible by Ernst Lobethall, a German Jew from Breslau, who worked alongside Avey at the Farben factory. Although fraternising was forbidden on pain of death, the two men became friends. “We spoke out of the corner of our mouths,” Avey says, “a difficult thing to do in German.”

He discovered that Lobethall had a sister, Susana, living in England. “I wrote to my mother, who told Susana that Ernst was alive. She posted 200 cigarettes to me via the Red Cross. Miraculously, four months later, they arrived. The cigarettes were worth a king’s ransom. Ernst suddenly became rich.”

With the cigarettes, Lobethall was able to buy boots and scraps of food that would later save his life. He also used them as bribes to help Avey to gain entrance to the Jewish camp.

“Despite the danger, I knew I had to bear witness,” Avey says. “As Albert Einstein said: the world can be an evil place, not because of those who do evil, but because of those who look on and do nothing. I’ve never been one to do nothing.”

The operation was planned meticulously. Avey found a Dutch Jew with a similar physique and persuaded him to exchange places for a day. Avey knew that they marched past each other at the same time every week. “The Nazis were rigid, you see,” he says. “To them orders were orders, to be carried out exactly. That was what allowed me to find a way round them.”

Avey shaved his head and blackened his face. At the allocated time, he and the Dutch Jew sneaked into a disused shed. There they swapped uniforms and exchanged places. Avey affected a slouch and a cough, so that his English accent would be disguised should he be required to speak.

“I joined the Stripeys and marched into Monowitz, a predominantly Jewish camp. As we passed beneath the Arbeit Macht Frei [work makes you free] sign, everyone stood up straight and tried to look as healthy as they could. There was an SS officer there, weeding out the weaklings for the gas. Overhead was a gallows, which had a corpse hanging from it, as a deterrent. An orchestra was playing Wagner to accompany our march. It was chilling.”

They were herded through the camp, carrying the bodies of those who had died that day. “I saw the Frauenhaus — the Germans’ brothel of Jewish girls — and the infirmary, which sent its patients to the gas after two weeks. I committed everything to memory. We were lined up in the Appellplatz for a roll call, which lasted almost two hours. Then we were given some rotten cabbage soup and went to sleep in lice-infested bunks, three to a bed.”

The night was even worse than the daytime. “As it grew dark, the place was filled with howls and shrieks. Many people had lost their minds. It was a living hell. Everyone was clutching their wooden bowls under their heads, to stop them getting stolen.” Lobethall had bribed Avey’s bedfellows with cigarettes. “They gave me all the details,” he says, “the names of the SS, the gas chambers, the crematoria, everything. After that, they fell asleep. But I lay awake all night.”

In the morning, Avey joined other prisoners for a roll call, followed by “breakfast” — a husk of black bread with a scrape of fetid margarine. “It wasn’t enough to sustain life. Everything was designed to make you waste away.” They were formed into groups and marched out of the camp, again to the accompaniment of an orchestra.

“When we passed the shed again, I slipped in to meet the Dutch Jew,” he says. “That was hair raising. Although I trusted him, I couldn’t be sure that he’d turn up. And if an SS officer had looked in the wrong direction at the wrong time, that would have been it.”

The changeover went smoothly, and Avey returned to the PoW camp. “The Dutch Jew perished, but I’m certain that this short reprieve prolonged his life by several weeks,” he says. “Whether that was a good thing, I don’t know.”

In 1945, as the Soviet Army closed in, the Nazis abandoned the camp and herded 60,000 prisoners in the direction of Germany, in what would become known as one of Death Marches. Avey, who by then was suffering from tuberculosis, was among them. Around 15,000 prisoners died on the way. “The road was littered with corpses,” he says. “I saw a chance to escape and seized it.”

He found his way to Allied lines and was transported back home. Two days before VE Day, he arrived at his parents’ Essex farm half-dead with exhaustion and sickness. They had not expected to see him again.

If Avey’s story still sounds implausible, there is no doubt about the help he gave to Lobethall. Last year the BBC screened a moving documentary, during which Avey learnt for the first time that his old friend had survived the war and died in New York in 2001. Before his death, Lobethall recorded a video testimony for Steven Spielberg’s Shoah Foundation, during which he emotionally recounts how his life was saved by Avey’s initiative and Susana’s cigarettes. This is the only moment that I see Avey’s steely façade falter.

“I was hospitalised for two years after the war,” Avey continues. “In 1947, I went to the military authorities to submit my information about Auschwitz. Their eyes glazed over. I wasn’t taken seriously. I was shocked, especially after the risks I’d taken. I felt completely disillusioned, and traumatised as well. So from then on I bottled it up, and tried to piece my life back together.”

Sir Martin Gilbert says: “By 1947, the trials of Nazi war criminals had been and gone. The war was over and people just wanted to get on with their lives. There was a whole mind-set of not really wanting to know what had happened any more. Many people had stories that nobody was interested in. It must have been very painful.”

Readjusting to normal life was hard. Avey became addicted to adrenalin, racing fast cars, travelling to Spain for the running of the bulls. He was plagued by nightmares and flashbacks. Even today he shows signs of trauma. He always carries an expensive gold watch, so that “if ever I find myself in a fix again, I’ve got something to fall back on”.

Sixty-five years after the liberation of Auschwitz, when eyewitnesses are dying out and Holocaust denial is burgeoning, Denis Avey’s extraordinary tale has finally found its moment. “I’m talking to you so it will do some good,” he says fiercely, pounding his fingers on the table for emphasis. “That’s all I’ve ever wanted.”